This invention relates to porous polymer structures and a method of preparing the same. More particularly, this invention relates to microporous polymer structures that may be readily prepared and are characterized by relatively homogeneous, three-dimensional, cellular microstructures and to a unique, facile process for preparing microporous polymer structures.
Several widely differing techniques have been previously developed for preparing microporous polymer structures. Such techniques range from what is termed, in the art, classical phase inversion, to nuclear bombardment, to incorporation of microporous solid particles in a substrate which are subsequently leached out, to sintering microporous particles together in some fashion. Prior efforts in the field have entailed still other techniques as well as innummerable variations of what may be considered as the classical or basic techniques.
The interest in microporous polymer products has been engendered by the numerous potential applications for materials of this type. These potential applications are well known and range from ink pads, or the like, to leather-like breathable sheets, to filter media. Yet, with all of the potential applications, the commercial usage has been relatively modest. And, the techniques being commercially utilized have various limitations which do not allow the versatility required to expand the applications to reach the potential market for microporous products.
As mentioned, some commercially available microporous polymer products are made by a nuclear bombardment technique. Such a technique is capable of achieving a rather narrow pore size distribution; however, the pore volume must be relatively low (i.e.--less than about 10% void space) to ensure that the polymer will not be degraded during preparation. Many polymers cannot be utilized in such a technique due to the lack of the ability of the polymer to etch. Still further, the technique requires that a relatively thin sheet or film of the polymer be used and considerable expertise must be employed in carrying out the procedure to avoid "double tracking", which results in the formation of oversized pores.
Classical phase inversion has also been commercially utilized to form microporous polymers from cellulose acetate and certain other polymers. Classical phase inversion has been reviewed in great detail by R. E. Kesting in SYNTHETIC POLYMERIC MEMBRANES, McGraw-Hill, 1971. In particular at page 117 of said reference it is explicitly stated that classical phase inversion involves the use of at least three components, a polymer, a solvent for said polymer and a non-solvent for said polymer.
Reference may also be made to U.S. Pat. No. 3,945,926 which teaches the formation of polycarbonate resin membranes from a casting solution containing the resin, a solvent, and a swelling agent and/or a nonsolvent. It is stated at lines 42-47, column 15, of said patent that in the complete absence of a swelling agent phase inversion usually does not occur and that with low concentrations of swelling agents, structures possessing closed cells are encountered.
From the foregoing discussion it is quite apparent that classical phase inversion requires the use of a solvent for the system at room temperature so that many other useful polymers cannot be substituted for the polymers such as cellulose acetate. Also from the process standpoint, the classical phase inversion process will generally be restricted to the formation of films due to the large amount of solvent used in the preparation of solutions which must be subsequently extracted. It is also apparent that classical phase inversion requires a relatively high degree of process control to obtain structures of desired configuration. Thus the relative concentrations of solvent, nonsolvent, and swelling agent must be critically controlled, as discussed in column 14-16 of U.S. Pat. No. 3,945,926. Conversely, to alter the number, size, and homogeneity of the resultant structure, one must modify the aforementioned parameters by trial-and-error.
Other commercially available microporous polymers are made by sintering microporous particles of polymers ranging from high density polyethylene to polyvinylidene fluoride. However, it is difficult with such a technique to obtain a product with the narrow pore size distribution required for many applications.
A still further general technique which has been the subject of considerable prior effort involves heating a polymer with various liquids to form a dispersion or solution and thereafter cooling, followed by removal of the liquid with a solvent or the like. This type of process is disclosed in the following United States patents which are only representative and not cumulative U.S. Pat. Nos.: 3,607,793; 3,378,507; 3,310,505; 3,748,287; 3,536,796; 3,308,073; and 3,812,224. It is not believed that the foregoing technique has been utilized commercially to any significant extent, if at all, probably due to the lack of economic feasibility of the particular processes which have previously been developed. Also, the prior processes do not allow the preparation of microporous polymers which combine relatively homogeneous microcellular structures with the pore size and pore size distributions which are typically desired.
With respect to the microporous polymers obtained by prior art techniques, no process known heretofore has been capable of yielding isotropic olefinic or oxidation polymers which have the major portion of pore sizes in the range of about 0.1 to about 5 microns while having a relatively narrow pore size ditribution, thus exhibiting a high degree of pore size uniformity throughout a sample thereof. Some prior art olefinic or oxidation polymers have had pore sizes in the foregoing range, but without a relatively narrow pore size distribution, thus making such materials without significant value in application areas, such as filtration, which require a high degree of selectivity. Furthermore, prior microporous olefinic or oxidation polymers which may be considered to have relatively narrow pore size distributions have had absolute pore sizes which are outside the aforementioned range, usually having substantially smaller pore sizes, for use in application areas such as ultra-filtration. Finally, some prior art olefinic polymers have had pore sizes in the foregoing range and what may be considered to be relatively narrow pore size distributions. However, such materials have been made by use of techniques, such as stretching which impart a high degree of orientation to the resultant anisotropic material, rendering it undesirable for many application areas. There thus has existed a need for microporous olefinic and oxidation polymers having a pore size in a range of from about 0.1 to about 5 microns and characterized as having a relatively narrow isotropic pore size distribution.
Also, a major drawback of many microporous polymers available heretofore has been the low flow rate of such polymers when used in structures such as microfiltration membranes. One of the major reasons for such low flow rates is the typically low void volume of many such polymers. Thus, perhaps 20 percent of the polymer structure, or less, may be "void" volume through which a filtrate may flow, the remaining 80 percent of the structure being the polymer resin which forms the microporous structure. Thus, there has also existed a need for microporous polymers having a high degree of void volume, especially with respect to olefinic polymers.
The copending Castro and Stoll application, previously identified herein, discloses a highly advantageous method for converting a particular type of liquid amine antistatic agent to a material which behaves as a solid. The advantages in processing which result are real and significant. It would be similarly beneficial to be able to convert other useful functional liquids such as flame retardants and the like to materials which behave as solids.
It is accordingly an object of the present invention to provide microporous polymer products characterized by relative homogeneity and narrow pore size distributions.
Another object is to provide a facile process which allows the economic production of microporous polymers.
A still further object lies in the provision of a process for making microporous polymer products, which has applicability to a wide number of useful thermoplastic polymers. A related and more specific object is to provide such a process which is capable of readily forming microporous polymers from any synthetic thermoplastic polymer including polyolefins, condensation polymers and oxidation polymers.
Yet another object of this invention is to provide microporous polymers in structures ranging from thin films to relatively thick blocks. A related object is to provide the ability to form microporous polymers in intricate shapes.
A further object is to provide the conversion of functional liquids to materials which possess the characteristics of a solid.